This invention relates to centre pivot irrigators.
Centre pivot irrigators have become widely accepted in farming practice. Typically, a centre pivot irrigator comprises a single pipe or, perhaps more often, a number of rigid pipes joined together end to end. In either case, the pipe or pipes form an elongate boom. The boom has an inner end anchored to a turret that is fixed in the ground and is designed in such manner, discussed below, that the boom is able to pivot about a fixed pivotal axis located at the turret. As it pivots, the boom is supported above the ground by the turret at the inner end and by pairs of ground wheels carried on triangulated supporting frames mounted one on each pipe. The ground wheels are driven in tandem by suitable means such as electric or hydraulic motors, causing the boom to pivot. When the boom has pivoted through 360°, it has passed over a circular piece of land. During this movement, irrigation water is pumped into the pipes making up the boom from a water supply pipe in the turret and discharged onto the land through outlets located at intervals along the pipes.
In this specification and the claims, the terms “pivoting” and “rotating” are used interchangeably when used with reference to the boom and, to avoid repetition, can be taken to mean “pivoting or rotating about the pivotal axis”.
Centre pivot irrigators vary in size from relatively small to quite large. For example, in the test prototype for the present invention, the boom comprises 11 pipes each of 50 meters in length. The area irrigated by the prototype is thus about 95 hectares. The scope of the present invention is not limited by the size of the irrigator nor to the type just described.
The above description is highly simplified. Commercially available centre pivot irrigators are provided with many sophisticated features that will not be described in more detail herein except where such detail is relevant to an understanding of the present invention.
In all centre pivot irrigators known to the applicant, each of the wheels remains at a substantially fixed distance from the pivotal axis as the boom rotates. This arises from the characteristic that the supporting frames and hence the wheels are mounted in fixed positions on the pipes and that the pipes are constrained to stay in a substantially straight line as the boom rotates. As a result, each wheel passes over the same piece of ground repeatedly as the boom rotates and it is well known that this can seriously damage or even destroy any crop growing in the path of the wheels. Furthermore, the land traversed by the wheels tends to deteriorate, forming channels and ruts. The channels and ruts can become deep enough to prevent the passage of tractors, mowers and other farm machinery as well livestock and the irrigator itself during subsequent passes. The ruts can impose considerable strain on the gearboxes, drive shafts and other components of the irrigator as the wheels have to repeatedly lift the irrigator up and out of potholes within the ruts. The ruts also can also cause damage to vehicles such as farm bikes and their riders because the ruts are often deep and almost invisible until the rider is on top of them. So the ruts become a safety issue. They also interfere with the run off of water from the land, creating boggy patches that are made even worse by subsequent passes of the irrigator.
Attempts have been made in the past to address these disadvantages. These attempts have included the laying of rocks, concrete or the like in the path of the wheels. This is expensive. When it is considered that each strip of land that is affected by the wheels can, in the experience of the applicant, be up to 2 meters wide, the cost to the farmer is significant. Other attempts include the fitting of so-called “boom backs” which spray the water out behind the irrigator with the intention that the ground on which the wheels run will be drier. Many manufacturers have recommended special “turf” tyres in soft ground applications. The Valley company of the USA offers a “third wheel” option to reduce the load on the two wheels mounted on each supporting frame.